Manganese dioxide and N,N′,N″-trihydroxyisocyanuric acid: A novel and recyclable catalytic system for aerobic oxidation of toluene derivatives in PEG-1000-based dicationic acidic ionic liquid

Article abstract of DOI:10.1002/aoc.3285

N,N′,N″-trihydroxyisocyanuric acid (THICA) with MnO₂ promotes the aerobic oxidation of toluene derivatives to corresponding acids in PEG-1000-based dicationic acidic ionic liquid (PEG₁₀₀₀-DAIL). It is demonstrated that THICA/MnO₂ is very active and selective and several toluene derivatives are efficiently oxidized to corresponding acids under mild conditions. Both the catalyst and PEG₁₀₀₀-DAIL can be reused after simple separation. A plausible mechanism is also proposed based on the experimental observations.

Full text of DOI:10.1002/aoc.3285

Full Paper
Received: 25 November 2014
Revised: 23 December 2014
Accepted: 7 January 2015
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3285
Manganese dioxide and N,N′,N″-
trihydroxyisocyanuric acid: a novel and
recyclable catalytic system for aerobic oxidation
of toluene derivatives in PEG-1000-based
dicationic acidic ionic liquid
Tingting Lua*, Lijie Zhanga, Zhongxue Gea, Yueping Jia and Ming Lub
N,N′,N″-trihydroxyisocyanuric acid (THICA) with MnO2 promotes the aerobic oxidation of toluene derivatives to corresponding
acids in PEG-1000-based dicationic acidic ionic liquid (PEG1000–DAIL). It is demonstrated that THICA/MnO2 is very active and selec-
tive and several toluene derivatives are efficiently oxidized to corresponding acids under mild conditions. Both the catalyst and
PEG1000–DAIL can be reused after simple separation. A plausible mechanism is also proposed based on the experimental observa-
tions. Copyright © 2015 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web-site.
Keywords: oxidation of toluene derivatives; THICA/MnO2; PEG1000–DAIL
In the study reported here, selecting various alkylbenzenes, the
performance of efficient and inexpensive MnO2 in combination
Introduction
The aerobic selective oxidation of toluene derivatives is one of the
most challenging subjects in organic catalysis, and green, efficient
and selective oxidation techniques are highly sought in the chemical
and pharmaceutical industry.[1] The traditional alkylaromatic oxidation
with oxygen, which is referred to as autoxidation, has its challenges
such as harsh reaction conditions, low energy efficiencies and limited
selectivity.[2] In last few decades, the valuable carbon radical-
producing catalyst N-hydroxyphthalimide (NHPI) has been recognized
as an effective catalyst for the aerobic oxidation of alkylaromatics. Un-
fortunately, this catalyst suffers from certain drawbacks such as low
conversion for the oxidation of electron-withdrawing substituted
alkylbenzenes.[3] Recently, another good carbon radical-producing
catalyst, N,N′,N″-trihydroxyisocyanuric acid (THICA), has been widely
applied in the catalytic oxidation of various alkylbenzenes, espe-
cially electron-withdrawing substituted alkylbenzenes. However,
the complex synthesis approach and corrosive acetic acid solvent
have limited the use of THICA in the oxidation.[4]
Investigations of ionic liquids, which have emerged as a potential
replacement for organic solvents or catalysts, are some of the most
active areas in both academic and industrial research owing to
their negligible volatility, remarkable solubility and potential
recyclability.[5] Recently, some novel temperature-dependent ionic
liquids (such as fluorous ionic liquids or polyether-based ionic liq-
uids) have been reported due to their advantages such as the re-
covery of catalyst with high efficiency. Among these ionic liquids,
PEG-1000-based dicationic acidic ionic liquid (PEG1000–DAIL), which
exhibits a temperature-dependent phase behavior with toluene, is
widely employed in various reactions (Scheme 1). PEG1000–DAIL
was attempted as the solvent in the oxidation of alkylbenzenes cat-
alyzed by THICA, and the results were good.[6]
with THICA was investigated in the aerobic oxidation with
PEG1000–DAIL as solvent. Both the catalyst and PEG1000–DAIL can
be successfully recovered and reused.
Experimental
Materials and Methods
All starting materials were purchased from commercial sources and
1
used without further treatment. H NMR (500 MHz) and 13C NMR
(125 MHz) spectra were recorded using a Bruker 500 spectrometer
with tetramethylsilane as an internal standard. Electron paramag-
netic resonance (EPR) spectra were obtained with a Bruker EMX-
10/12 spectrometer. Infrared (IR) spectra were recorded with a
Bruker Vector 22 IR spectrometer, as KBr pellets. High-performance
liquid chromatography (HPLC) experiments were performed with a
liquid chromatograph (Dionex Softron GmbH, USA). The conver-
sions of the substrates and the selectivities of products were esti-
mated from the peak areas based on the internal standard
technique. The products were determined in some cases by com-
parison of their HPLC patterns with those of authentic samples.
*
Correspondence to: Tingting Lu, Xi’an Modern Chemistry Research Institute, Xi’an
710065, China. E-mail: tt252525@gmail.com
a Xi’an Modern Chemistry Research Institute, Xi’an 710065, China
b School of Chemical Engineering, Nanjing University of Science and Technology,
Nanjing, China
Appl. Organometal. Chem. (2015)
Copyright © 2015 John Wiley & Sons, Ltd.

T. Lu et al.
the results of the oxidation over the THICA/MnO2 catalytic system in
various solvents. After a series of experiments, PEG1000–DAIL proves
to be the best solvent. And in traditional solvent acetic acid or ace-
tonitrile the conversion and selectivity are both lower under com-
parable conditions.[8] Poor conversion is obtained when the
reaction is performed in dichloromethane.[9] The catalytic activities
of THICA are not encouraged in PEG-1000-based dicationic ionic liq-
uid (PEG1000–DIL) due to its lack of acidity. Other PEG-based dica-
tionic acidic ionic liquids were also tested, but lower conversions
and selectivities are obtained.
In order to obtain further insight into the reaction under investi-
gation, the influence of reaction time and temperature on the cat-
alytic performance of THICA/MnO2 was investigated (Fig. 1). The
conversion or selectivity curves as a function of reaction time are
similar no matter which temperature was used. The reaction speed
sharply increases in initial 8 h, and then the oxidation proceeds
smoothly during 8–12 h, but the conversion and selectivity are
practically steady at 99% after 12 h. A reaction time of 12 h is con-
sidered as a preferable reaction time. The reaction temperature was
Scheme 1. Preparation of PEG1000–DAIL.
General Procedure for Oxidation
THICA was prepared using a literature procedure.[7] 1H NMR (DMSO-
d6, δ, ppm): 11.03 (s, 3H). 13C NMR (DMSO-d6, δ, ppm): 146.6. IR
(cmÀ1): 3539, 3166, 2827, 1720, 1433, 1211, 1010, 701.
PEG1000–DAIL was prepared using a procedure given in the liter-
ature (Scheme 1).[16] 1H NMR (D2O, δ, ppm): 2.15 (t, 4H, J = 7 Hz, 2 ×
CH2), 2.77 (m, 4H, 2 × CH2), 3.45–3.66 (m, 90.3H, (OCH2CH2)n), 3.74
(4H, 2 × CH2), 4.21–4.30 (8H, 4 × NCH2), 7.41 (s, 4H, 4 × CH), 8.71
(s, 2H, 2 × CH). 13C NMR (D2O, δ, ppm): 25.2, 47.7, 47.9, 49.1, 68.6,
68.7, 69.6, 122.3, 123.1, 136.0. IR (cmÀ1): 3849, 3400, 3020, 2883,
1732, 1591, 1456, 1431, 1377, 1122, 1109, 972, 883, 798, 677, 577.
The substrate (5 × 10À3 mol), PEG1000–DAIL (2.8 × 10À5 mol),
THICA (3.0 × 10À4 mol, 6 mol%) and MnO2 (2.0 × 10À4 mol, 4 mol
%) were placed in a three-necked flask. Oxygen was bubbled into
the flask at a flow rate of 20 ml minÀ1. The reaction mixture was
stirred at a specific temperature for a specific time, and the reaction
progress was monitored using HPLC. After completion of the reac-
tion, the mixture was cooled to room temperature and extracted
with ether a few times. After concentration of the ether solution,
the products were taken for HPLC measurement and PEG1000–DAIL
was reused without any treatment.
Table 1. Oxidation of toluene with various amounts of THICA/MnO2a
Entry
THICA
(mol%)
MnO2
(mol%)
Conversion
(%)b
Selectivity
(%)b
1
2
3
4
5
6
7
8
9
6
6
0
3
3
6
8
6
8
4
0
4
2
4
2
4
6
6
98.8
50.6
30.2
42.6
67.1
74.3
99.0
98.8
99.1
99.0
58.1
52.4
64.1
90.4
82.5
99.0
99.2
99.0
aReaction conditions: 5 × 10À3 mol toluene, 2.8 × 10À5 mol PEG1000
DAIL, 80°C, 12 h.
bConversion and selectivity of products were estimated from the peak
–
areas based on the internal standard technique using HPLC.
Results and Discussion
Influence of Reaction Conditions on Oxidation of Toluene with
THICA/MnO2
Table 2. Influence of solvent in oxidation catalyzed by THICA/MnO2a
The initial oxidation of toluene as a model substrate was investi-
gated using various amounts of THICA and MnO2 in PEG1000–DAIL
at 80°C for 12 h (Table 1). In the study, 6 mol% THICA and 4 mol%
MnO2 exert a better effect on the oxidation, affording 98.8% con-
version and 99.0% selectivity of benzoic acid (Table 1, entry 1). In
sharp contrast, the conversion is 50.6% when only using THICA as
catalyst. And MnO2 alone, realizing 30.2% conversion, is of low effi-
ciency (Table 1, entries 2 and 3). In comparison with the reaction
with 6 mol% THICA and 4% MnO2, the conversion of toluene mark-
edly decreases when the amounts of both catalytic components
are reduced by half. And the conversion is less than 75% when
the amount of one component is decreased to half in spite of
the other component being unchanged (Table 1, entries 4–6). On
the other hand, as the amount of either one component or both
the components is 1.5 times that of the original, the reaction
reaches a plateau at about 99.1% conversion and 99.2% selectivity
(Table 1, entries 7–9).
Entry
Solvent
Conversion (%)b
Selectivity (%)b
1c
2d
3d
4d
5c
6c
7c
8c
9c
10c
PEG1000–DAIL
CH2Cl2
98.8
61.4
96.5
88.3
90.2
97.1
97.7
85.5
25.8
11.6
99.0
52.2
95.1
87.6
89.0
98.3
98.5
90.4
43.9
32.3
HOAc
CH3CN
PEG1000–DIL
PEG400–DAIL
PEG600–DAIL
PEG2000–DAIL
[EMIm][Pro]
[BMIm]BF4
aReaction conditions: 5 × 10À3 mol toluene, 3.0 × 10À4 mol THICA, 2.0 ×
10À4 mol MnO2, 80°C, 12 h.
bConversion and selectivity of products were estimated from the peak
areas based on the internal standard technique using HPLC.
cSolvent: 2.8 × 10À5 mol.
dSolvent: 5 ml.
It is known from previous studies that the solvent has a great in-
fluence on the catalytic performance of THICA. Table 2 summarizes
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Appl. Organometal. Chem. (2015)

MnO2/THICA-catalyzed oxidation of toluene
also studied (Fig. 1). When the oxidation occurs at 70°C, low conver-
sion and selectivity are obtained due to incomplete dissolution of
toluene in PEG1000–DAIL. However, the conversion and selectivity
still decrease with higher temperature (90°C), a reason being that
much toluene may be exhausted by evaporation.
Table 3. Selective oxidation of substrates with THICAa
Oxidation of Various Substrates
To study the scope of the THICA/MnO2 catalytic system, the efficacy
of transformation of substituted toluenes to carboxylic acids was
investigated (Table 3). When electron-rich toluene derivatives are
used as substrates, the reactions run smoothly with excellent
conversions and selectivities. For example, xylenes are selectively
oxidized to diacids, and high selectivity of p-tert-butylbenzoic acid
is obtained even for the sterically hindered p-tert-butyltoluene.
The reactivity of electron-withdrawing substituted toluene is also
demonstrated. As for the oxidation of toluene containing generic
electron-poor group, the THICA/MnO2 catalysis system also
exhibits quite a high catalytic activity. However, the scope of
the methodology is found to be rather narrow in terms of strong
electron-withdrawing substituted toluene, being unsatisfactory for
p-nitrotoluene and o-nitrotoluene.
Run
R
Time (h) Conversion Products and selectivity (%)b
(%)b
Main product Side product
1
4-CH3
2-CH3
3-CH3
4-C(CH3)3
4-NH2
4-OMe
4-CN
15
15
15
13
11
13
13
13
14
14
14
17
20
93.8
93.6
88.9
80.1
94.7
91.5
92.6
90.5
93.1
92.9
93.6
81.3
58.4
88.9
85.0
80.5
93.4
95.6
94.4
95.3
92.7
94.1
92.6
94.2
88.9
40.5
10.4
12.2
16.6
4.1
2
3
4
5
3.8
6
5.0
7
3.2
8
4-COOH
4-Cl
5.9
9
4.4
Reaction Mechanism
10
11
12
13
2-Cl
6.6
4-Br
4.1
Yang and co-workers have done in-depth research on the catalytic
mechanism of NHPI with MnO2, and they suggested MnO2 and
other Mn species formed in situ could oxidize NHPI to
phthalimide-N-oxyl.[10] Ishii et al. indicated that the catalytic mech-
anism of THICA is a free radical reaction, which is similar to that of
NHPI.[4] And various radical initiators of NHPI (such as HNO3 and
dimethylglyoxime) were successful as radical initiators of THICA in
previous work.[11]
4-NO2
2-NO2
10.2
42.7
aReaction conditions: 5 × 10À3 mol toluene, 3.0 × 10À4 mol THICA, 2.0 ×
10À4 mol MnO2, 2.8 × 10À5 mol PEG1000–DAIL, 80°C.
bConversion and selectivity of products were estimated from the peak
areas based on the internal standard technique using HPLC.
In order to obtain further information on the role of the MnO2
species in the oxidation under investigation, EPR measurements
were carried in PEG1000–DAIL. Due to the limits of the analytical
condition, the PEG1000–DAIL solution was examined only at room
As a consequence, a plausible reaction pathway for the aerobic
oxidation of toluene using a combination of THICA and MnO2 is il-
lustrated in Scheme 2. The first step of the reaction involves the cor-
responding radical of THICA, which is formed by the reaction of
THICA and Mnn+ (MnO2 or Mn3+). Then toluene undergoes abstrac-
tion of a hydrogen atom by the radical of THICA, to form benzyl rad-
ical. The resulting benzyl radical traps oxygen to form benzylperoxy
radical, whose hydrogen abstraction produces benzyl hydroperox-
ide. The resulting benzyl hydroperoxide can be decomposed by
Mn(nÀ1)+ (Mn2+ or Mn3+), and then oxidized further to produce
benzoic acid.
temperature. When both MnO2 and THICA are added in PEG1000
–
DAIL, an ESR signal based on the THICA radical (g = 2.00070, A (N)
= 4.85 G)[24] is observed although it is very weak (Fig. 2). However,
no signal appears in PEG1000–DAIL containing only THICA. It is
shown that MnO2 could be the radical initiator of THICA.
Figure 1. Effect of time and temperature on oxidation. (Reaction
conditions: 5 × 10À3 mol toluene, 3.0 × 10À4 mol THICA, 2.0 × 10À4 mol
MnO2, 2.8 × 10À5 mol PEG1000ÀDAIL).
Figure 2. EPR spectrum of the THICA radical.
Appl. Organometal. Chem. (2015)
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T. Lu et al.
in the conversion of toluene are observed. However, the selectivity
of benzoic acid decreases obviously after seven reuses.
Conclusions
We have shown that THICA/MnO2 can be used as a catalyst for
the aerobic oxidation of alkylaromatics in PEG1000–DAIL. Toluene
derivatives with electron-donating or electron-withdrawing
groups were oxidized to their corresponding acids in moderate
to excellent conversions. Both the catalyst and PEG1000–DAIL
were recycled several times without any further addition. This
methodology avoids the use of toxic solvents or over-
stoichiometric amounts of oxidants, showing good economic
and environmental advantages.
Scheme 2. Possible reaction mechanism for the catalytic cycle of the
THICA/MnO2 system.
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Reusability
The reusability of PEG1000–DAIL and catalyst (THICA/MnO2) was in-
vestigated under identical conditions as described in Table 1.
PEG1000–DAIL can be separated from the reaction mixture by the
addition of ether. The upper organic phase with the product is
separated from PEG1000–DAIL with the catalyst using simple
decantation. Then PEG1000–DAIL is used for another consecutive
run without further treatment. The reusability results are depicted
in Fig. 3. Upon eight consecutive reuses, no noticeable variations
Supporting Information
Additional supporting information may be found in the online ver-
sion of this article at the publisher’s web-site.
wileyonlinelibrary.com/journal/aoc
Copyright © 2015 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2015)